1. Field of the Invention
Embodiments of the present invention generally relate to drilling and completion of oil and gas wells. More specifically, embodiments of the present invention relate to methods and apparatus for forming a wellbore by drilling with casing. Embodiments of the present invention generally relate, more particularly, to the construction of lateral wellbores.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed in a formation using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and the drill bit are removed, and the wellbore is typically lined with a string of pipe called casing. The casing forms a major structural component of the wellbore and serves several important functions, such as preventing the formation wall from caving into the wellbore, isolating different zones in the formation, preventing the flow of fluids into the wellbore, and providing a means of maintaining control of fluids and pressure while drilling. Casing is available in a range of sizes and material grades, the choice of which is typically determined by a particular application.
The casing typically extends down the wellbore from the surface to a designated depth. Various downhole tools are often run through the casing to perform various operations downhole in the wellbore. Accordingly, the drift diameter of the casing dictates the types of downhole tools that may be run through the casing. Drift diameter generally refers to the inside diameter that the casing manufacturer guarantees per specifications. In other words, the drift diameter may be used (e.g., by a well planner) to determine what size tools may later be run through the casing.
For various production oriented reasons, it may be desirable to form a lateral (e.g., deviating from vertical) wellbore extending from a main (or “parent”) wellbore. For example, because a lateral wellbore typically penetrates a greater length of the reservoir, it may offer significant production improvement over a purely vertical main wellbore. Lateral wellbores extending from a cased main wellbore may be formed by removing a portion of the main wellbore casing to expose a portion of the formation. The lateral wellbore may then be formed by drilling out from the main wellbore through the exposed portion of the formation. Various well-known techniques are available to achieve the desired deviation from the main wellbore when drilling the lateral wellbore.
For the previously described reasons (e.g., support, isolation, etc.), it is also desirable to line a lateral wellbore with casing. However, in order to reach the lateral wellbore, casing used to line the lateral wellbore must pass through the main wellbore casing. Therefore, to run the casing into the lateral wellbore, the outer diameter of the casing used to line the lateral wellbore must be smaller than the inner diameter of the main wellbore casing. Accordingly, casing used to line conventional lateral wellbores has been limited to casing having inner diameters significantly smaller than the main wellbore casing. As a result of this smaller inner diameter, the types of downhole tools that may be run in the lateral wellbore are typically restricted, thereby limiting the types of operations that may be performed therein. Accordingly, what is needed is an improved method for forming a lateral wellbore lined with casing having an enlarged inner diameter relative to casing lining conventional lateral wellbores.
To drill within the wellbore to a predetermined depth in conventional well completion operations, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annular area is thus formed between the string of casing and the formation. The casing string is temporarily hung from the surface of the well. A cementing operation is then conducted in order to fill the annular area with cement. Using apparatus known in the art, the casing string is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a first designated depth with a drill bit on a drill string. The drill string is removed. A first string of casing or conductor pipe is then run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed, or “hung” off of the existing casing by the use of slips which utilize slip members and cones to wedgingly fix the new string of liner in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
As an alternative to the conventional method, drilling with casing is a method sometimes used to place casing strings within the wellbore. This method involves attaching a cutting structure in the form of a drill bit to the same string of casing which will line the wellbore. Rather than running a drill bit on a smaller diameter drill string, the drill bit or drill shoe is run in at the end of the larger diameter of casing that will remain in the wellbore and be cemented therein. Drilling with casing is a desirable method of well completion because only one run-in of the working string into the wellbore is necessary to form and line the wellbore for each casing string.
Specifically, drilling with casing is typically accomplished by lowering and rotating a first casing string with a cutting structure attached thereto into a formation to form a portion of the wellbore at a first depth. During the lowering of the casing string, it is often necessary to circulate drilling fluid while drilling into the formation to form a path within the formation through which the casing string may travel. The first casing string is cemented into the formation. Next, a second casing string with a drill bit attached thereto is lowered and rotated into the formation while circulating fluid to form a portion of the wellbore at a second depth. The second casing string is hung off of the first casing string and cemented into the formation. This process can be repeated with additional casing strings until the wellbore extends to the desired depth.
Because the second casing string must travel through the first string of casing to reach the formation below the first casing string, the second casing string must have a smaller inner diameter than the second casing string. Historically, therefore, as more casing strings were set in the wellbore, the casing strings became progressively smaller in diameter in order to fit within the previous casing string. The drill bit for drilling to the next predetermined depth must thus become progressively smaller as the diameter of each casing string decreases in order to fit within the previous casing string. Therefore, multiple drill bits of different sizes are ordinarily necessary for drilling in well completion operations. Progressively decreasing the diameter of the casing strings with increasing depth within the wellbore limits the size of wellbore tools which are capable of being run into the wellbore. Furthermore, restricting the inner diameter of the casing strings limits the volume of hydrocarbon production which may flow to the surface from the formation.
Recently, methods and apparatus for expanding the diameter of casing strings within a wellbore have become feasible. When using expandable casing strings to line a wellbore, the well is drilled to a first designated depth with a drill bit on a drill string, then the drill string is removed. A first string of casing is set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing is run into the drilled out portion of the wellbore at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. Cement can be placed behind the second casing string and then the second casing string is expanded into contact with the existing first string of casing with an expander tool. This process is typically repeated with additional casing strings until the well has been drilled to total depth.
An advantage gained with using expander tools to expand expandable casing strings is the decreased annular space between the overlapping casing strings. Because the subsequent casing string is expanded into contact with the previous string of casing, the decrease in diameter of the wellbore is essentially the thickness of the subsequent casing string. However, even when using expandable technology, casing strings must still become progressively smaller in diameter in order to fit within the previous casing string.
Currently, monobore wells are being investigated to further limit the decrease in the inner diameter of the wellbore with increasing depth. Monobore wells would theoretically result when the wellbore is approximately the same diameter along its length or depth through the expansion of casing strings, causing the path for fluid between the surface and the wellbore to remain consistent along the length of the wellbore and regardless of the depth of the well. In a monobore well, tools could be more easily run into the wellbore because the size of the tools which may travel through the wellbore would not be limited to the constricted inner diameter of casing strings of decreasing inner diameters.
Theoretically, in the formation of a monobore well, a first casing string could be inserted into the wellbore and cemented therein. Thereafter, a second casing string of a smaller diameter than the first casing string could be inserted into the wellbore and expanded to approximately the same inner diameter as the first casing string. The casing strings may be connected together through a conventional hanger, or by expanding the inner diameter of the larger diameter first casing string, which is located above the second casing string, where the first and second casing strings overlap. Additional casing strings would be inserted into the wellbore and expanded, as described in relation to the first and second casing strings, until the wellbore extends to the desired depth.
With monobore well investigation, certain problems present. One problem relates to the expansion of the smaller casing string into the larger casing string to form the connection therebetween. Current methods of expanding casing strings in a wellbore to create a connection between casing strings requires the application of a radial force to the interior of the smaller casing string and expanding its diameter out until the larger casing string is itself pushed past its elastic limits. The result is a connection having an outer diameter greater than the original outer diameter of the larger casing string. While the increase in the outer diameter is small in comparison to the overall diameter, there are instances where expanding the diameter of the larger casing string is difficult or impossible. For example, in the completion of a monobore well, the upper casing string may be cemented into place before the next casing string is lowered into the well and its diameter expanded. Because the annular area between the outside of the larger casing string and the borehole therearound is filled with cured cement, the diameter of the larger casing string cannot expand past its original shape. Expansion of the required magnitude may also rupture the casing.
When hanging a casing string from another casing string, whether during a drilling operation or a drilling with casing operation, the casing string being hung may be set mechanically or hydraulically. A typical apparatus for setting a casing string in a well casing includes a liner hanger and a running tool. The running tool is provided with a valve seat obstruction which will allow fluid pressure to be developed to actuate the slips in order to set the liner hanger in the well casing. Once the liner hanger has been set, the running tool is rotated counterclockwise to unscrew the running tool from the liner hanger and the running tool is then removed.
One advantageous use for expandable tubulars is to hang one tubular within another. For example, the upper portion of a casing string can be expanded into contact with the inner wall of a casing in a wellbore. In this manner, the bulky and space-demanding slip assemblies and associated running tools can be eliminated. One problem with using expandable tubular technology used casing strings relates to cementing the casing strings within the wellbore. Cementing is performed by circulating uncured cement down the wellbore and back up an annulus between the exterior of the casing string being set and the wellbore therearound. In order for the cement to be circulated, a fluid path is necessary between the annulus and the wellbore. Hanging a casing string in a wellbore by circumferentially expanding its walls into the well casing obstructs the juncture and prevents circulation of fluids. To avoid this circulation problem, casing strings must usually be temporarily hung in a wellbore prior to cementing.
Therefore, a need exists for a method and apparatus for forming a substantially monobore well when drilling with casing. There is a further need for an apparatus and method for use when drilling with casing for forming a cased wellbore with an inner diameter which does not decrease with increasing depth within the wellbore. There is a yet further need for an apparatus and method for use in drilling with casing which involves running a casing string of smaller inner diameter into a formation and subsequently expanding a casing string of larger inner diameter to form a wellbore with substantially the same inner diameter along its length.
Moreover, there is a need for apparatus and methods that permit casing to be hung in a well and also leave a fluid path around the casing, at least temporarily. Additionally, there is a need for casing having a means for circulating fluids therearound even after the casing has been hung within the wellbore or previously installed casing.